Bioartificial organs "BAO" are devices which contain living cells and are designed to provide a needed metabolic function to a host.
The cells encapsulated in BAOs supply one or more biologically active molecules to the host that may be used to prevent or treat many clinical conditions, deficiencies, and disease states.
For example, BAOs containing insulin secreting cells may be used to treat diabetes. Similarly other diseases such as hypoparathyroidism and anemia may be treated by using cells which secrete parathyroid hormone and erythropoietin, respectively.
Bioartificial organs may also be used to supply biologically active molecules for the treatment or prevention of neurodegenerative conditions such as Huntington's disease, Parkinson's disease, Alzheimer's disease, and Acquired Immune Deficiency Syndrome-related dementia. Additionally, lymphokines and cytokines may also be supplied by BAOs to modulate the host immune system. Other biologically active molecules which may be provided by bioartificial organs include, catecholamines, endorphins, enkephalins, and other opioid or non-opioid peptides that are useful for treating pain. Enzymatic deficiencies may also be treated by using BAOs. Alternatively, the biologically active molecule may remove or eliminate deleterious molecules from the host. For example, a BAO may contain cells which produce a biologically active molecule that can be used to "scavenge" cholesterol from a host.
Various "macrocapsule" BAOs are known. See, e.g., Aebischer (U.S. Pat. No. 5,158,881), Dionne et al. (WO 92/03327), Mandel et al. (WO 91/00119), Aebischer (WO 93/00128). BAOs also include extravascular diffusion chambers, intravascular diffusion chambers, intravascular ultrafiltration chambers, and microcapsules. See, e.g., Lim et al., Science 210:908-910 (1980); Sun, A. M., Methods in Enzymology 137: 575-579 (1988); Dunleavy et al. (WO 93/03901) and Chick et al. (U.S. Pat. No. 5,002,661).
Because the cells encapsulated in the BAO provide the needed metabolic function, it is desirable that those cells optimally supply the biologically active molecule that effects that function. Typically, differentiated, non-dividing cells may be preferred over dividing cells for use in BAOs because they allow for the optimal production of the desired biologically active molecule. For example, many differentiated, non-dividing cells produce a greater quantity of a desired therapeutic protein than dividing cells because the expression of differentiation specific genes and cell division are thought to be antagonistic processes. Wollheim, "Establishment and Culture of Insulin-Secreting .beta. Cell Lines," Methods in Enzymology, 192, p. 223-235 (1990). Cellular replication capacity decreases as cells differentiate. In many cases, proliferation and differentiation are mutually exclusive. Gonos, "Oncogenes in Cellular Immortalisation and Differentiation," 13, Anticancer Research, p. 1117 (1993).
The use of differentiated tissue is advantageous because the functional properties of tissue desired for incorporation into a BAO have most often been defined by the properties of differentiated tissue in vivo. Another advantage to the use of differentiated, non-dividing cells is that the cell number within the BAO will remain relatively constant. This, in turn, leads to more predictable results and stable dosage for the recipient host. Additionally, differentiated cells are better suited for use in BAOs which encapsulate more than one cell type secreting biologically active molecules. In such BAOs, if dividing cells are used, different cell types may grow at different rates, resulting in the overgrowth of one cell type. By using differentiated, non-dividing cells, the relative proportions of two or more synergistic cell types can be more readily controlled.
Although in many instances the use of differentiated cells is advantageous, there have been various problems associated with utilizing differentiated cells directly isolated from mammals.
First, there is the potential contamination of the isolated tissue which may require that the tissue taken from each animal be subjected to costly and time-consuming testing to assure that it is pathogen-free.
Second, tissue can be damaged during isolation due to the use of mechanical or enzymatic isolation procedures in the isolation process. The mechanical manipulations are not always easily standardized, resulting in variability between isolations.
Third, ischemia may occur during isolation causing tissue damage.
Fourth, reproducible yields may be difficult because of variations in tissue donors. For example, the age, sex, health, hormonal status of the source animal can affect the yield and quality of the tissue of interest.
Fifth, sometimes there is not enough source tissue to meet the projected demand for the BAO. This occurs for example, in a case where the source tissue comes from a small sized organ or where the ultimate need for tissue amounts is high. If the source of the isolated tissue is human, there is frequently a severe shortage of donor tissue.
Sixth, in some cases, it is desirable to genetically modify the cells used in the BAO. Non-dividing tissue to date has been difficult to genetically modify in vitro and the yields and properties of the modified cells may be uncertain. Thus, because of the foregoing problems, while the use of differentiated, non-dividing cells is desirable, a need exists for a method of producing and maintaining differentiated, non-dividing cells for encapsulation in BAOs.
Because of these problems, dividing cells and cell lines have been favored for use within BAOs to provide the needed biological function. One important advantage in using dividing cells is that such cells may be grown to large numbers in vitro and screened for pathogens and banked. This allows an almost unlimited supply of tissue for lower production costs. Selection schemes such as cell sorting or cloning may be applied to the cell bank to develop subpopulations with improved characteristics. Additionally, dividing cells and cell lines are more amenable to genetic engineering than differentiated, non-dividing cells. The ability to introduce heterologous recombinant DNA allows many new possibilities for the alteration of the function or phenotype of cells to be encapsulated in the BAO. This in turn provides for a greater diversity of therapeutic uses for BAOs.
However, as discussed supra, the disadvantages in encapsulating continuously dividing cells in a BAO include poor regulation of cell numbers in the device that may result in less predictability in production of the desired biologically active molecule.
While in most cases it may be desirable to limit or minimize cell growth within the BAO, in other cases, e.g., where the BAO is implanted in a "hostile" environment, it may be desirable to allow the cells to proliferate slowly to maintain cell numbers in the BAO.
There is another problem associated with encapsulating cells in general. A variety of cell types have cell adherent properties such that cells tend to adhere to each other and form dense agglomerations or aggregates, especially if there is no adequate substrate available for the cells. Such cell clusters may develop central necrotic regions due to the relative inaccessibility of nutrients and oxygen to cells embedded in the core, or due to the build up of toxic products within the core. The necrotic tissue may also release excess cellular proteins which unnecessarily flood the host with xeno-proteins or other factors which are detrimental to the surviving cells, e.g., factors which elicit a macrophage or other immune response. This problem may be exacerbated when cells are encapsulated in a BAO with a semipermeable membrane jacket because of diffusional constraints across the membrane. Often less oxygen and fewer host supplied nutrients are available within the BAO. In addition, waste products may accumulate in the BAO.
These dense cellular masses can form slowly into dense colonies of cell growth or form rapidly, upon the reassociation of freshly-dispersed cells or tissue mediated by cell-surface adhesion proteins. Cells or tissues with a high metabolic activity may be particularly susceptible to the effects of oxygen or nutrient deprivation, and die shortly after becoming embedded in the center of a large cell cluster. Many endocrine tissues, which normally are sustained by dense capillary beds, exhibit this behavior; islets of Langerhans appear to be particularly sensitive when encapsulated.
There is a need to have a method and composition for controlling the growth of encapsulated cells which combines the various advantages of both proliferating cells and differentiated, non-dividing cells. The present invention provides methods and compositions whereby cells can be proliferated and expanded indefinitely in vitro and where the balance between proliferation and differentiation can be controlled when the cells are encapsulated within the BAO so that the device performs in the desired manner. This invention thus allows regulation of the cell number within the BAO and may therefore provide improved regulation of the output level of the capsule. This invention also provides methods for controlling the growth of cells by controlling cell location within the BAO, thereby reducing the formation of undesirable necrotic cell cores in the BAO. Controlling the cell number and cell location within the BAO also provides the advantage of facilitating optimization of the BAO membrane and other device parameters to the particular encapsulated cell type. This is because the required device characteristics are more readily determined for a fixed cell population than for a dividing cell population in the BAO. Additionally, long term delivery of biologically active molecules can be achieved.